A classic TV sitcom makes any number of things that are difficult, unpredictable and uncontrollable look easy, meant to be and precisely engineered.

"The Mary Tyler Moore Show," the beloved comedy that debuted in 1970, is a perfect example of seemingly predestined comedy magic. The leading lady was funny and winning; the ensemble cast played off each other like musicians in a world-class orchestra; audience loyalty and awards rained down until the series went off on the air on a triumphant high note in 1977.

In telling the tale, Armstrong, a former Entertainment Weekly reporter, offers a breezy tour of the way things used to be in television, back when there were only three networks, ABC, CBS and NBC. The genesis for "The Mary Tyler Moore Show" was rooted in demographic data and changing fashions, volatile elements that continue to uproot schedules and shows to this day.

For CBS, after years of rating success with such cornball, rural-themed favorites as "The Beverly Hillbillies," "Petticoat Junction" and "Green Acres," the tide was turning. Programming executives at CBS were jealously eyeing NBC, which was pulling ahead in ratings among "young, wealthy, educated consumers," Armstrong writes.

So the door opened for "The Mary Tyler Moore Show," which was, in its day, a somewhat radical, sophisticated idea. Moore had become a viewer favorite playing a wife, a standard role for women in sitcoms. But Moore's Laura Petrie on "The Dick Van Dyke Show" was a witty and worthy addition to one of the best sitcoms in the history of TV.

Mary and Lou and Rhoda and Ted

Jennifer Keishin Armstrong

Simon & Schuster

$26, 336 pages

Moore had shown what she could do, so when it came time to build a show around her, the character of Mary Richards was to be a break from female leads of comedies past. She wasn't a wife, but a divorced woman; she wasn't a ditzy would-be actress who spent most of her time with her boyfriend, a la Marlo Thomas in "That Girl"; she was single, capable, independent and in no rush to get married.

We've seen that heroine in countless sitcoms since. But in 1970, the network balked. Divorced? The CBS executives thought people didn't like that word, let alone the concept. Worse yet, they worried viewers would think Moore had divorced Dick Van Dyke.

As a compromise, Mary Richards became a woman who was out of a longtime relationship and headed off to Minneapolis TV news station WJM to make it on her own, as the famous theme song had it.

Armstrong is at her best in the early sections of the book, sketching the context of an era when feminism was still a new idea and establishing the key players. Moore, who has written her own account of her life and career (including details more frank than what Armstrong glosses over, such as Moore's alcoholism, and her son's addiction and death), apparently didn't talk to Armstrong.

But Armstrong did interview others on the show's founding team, including co-creator/writers and executive producers James L. Brooks (who went on to direct "Terms of Endearment" and produce "The Simpsons") and Allan Burns; Valerie Harper (who played Mary Richard's best friend, Rhoda Morgenstern, and later starred in her own "Rhoda" spinoff); and Grant Tinker, Moore's former husband and co-founder of her namesake production company, MTM Enterprises.

It took a little while for all the pieces to click, for the writers to craft the roles to the actors' strengths and for the cast members -- including Ed Asner as gruff newsroom boss, Lou Grant; Ted Knight as egotistical news anchor Ted Baxter; Gavin MacLeod as wisecracking news writer Murray; and Cloris Leachman as Phyllis, Mary's irritating landlady -- to find their groove.

But by the time the first season was ending, "The Mary Tyler Moore Show" was humming, and making history, with its depiction of a single woman who was content with her life and surrounded by a family of friends, neighbors and colleagues.

Armstrong emphasizes the significant role played by women writers, including Treva Silverman, in setting the tone of the show and capturing facets of women's lives that the male staffers weren't as attuned to.

Once "The Mary Tyler Moore Show" is up and running, Armstrong's book starts to meander. She touches on other hit shows of the era, including "All in the Family," which was a bigger deal in its day but has aged less well than Moore's show. We also hear how the famous series finale -- with the station being sold, and everyone fired except for buffoonish Ted Baxter -- came about.

By that point in the book, there's not much Armstrong can write that can match the experience of actually watching old episodes of "The Mary Tyler Moore Show." Reruns are airing at 9 p.m. weeknights on the Me-TV Network.

Beyond that, the show's impact can still be felt and seen everywhere on TV, from "30 Rock" reruns, to HBO's "Girls," to "New Girl" on Fox. As the theme song said, "Love is all around." So are the TV children of "The Mary Tyler Moore Show."